SAN DIEGO -- For the cost of an additional $3.75 a month to local water bills, the region's residents could have a more reliable water supply through the year 2030, San Diego County Water Authority officials said Thursday.

As part of a long-term planning process, officials held a workshop in San Diego to discuss what a $2 billion 'master plan' would mean to the region's water consumers. The plan includes a seawater desalination plant in Carlsbad, a water treatment plant and increasing the storage capacity of the San Vicente Dam, among other projects.

An earlier estimate would have tacked an extra $11 onto the monthly water bills of all ratepayers in the county by 2020. Water Authority officials said the latest estimate is based on incremental rate increases as new projects come on line until 2016.

The new estimate is based on more reliable figures than earlier projections, officials said.

"When they came out with that estimate, it was the best-guess estimate," said Karen Brust, the agency's director of finance. "This (latest estimate) has gone through a very drastic analysis."

Members of the Water Authority board chose the plan centered on seawater desalination last year as a way of insuring regional water supplies well into the future. The desalination plant would provide up to 86,000 acre-feet of drinking water by 2015. That is about 10 percent of the estimated water use in 2016, officials said.

The plant would also mean less dependency on imported water.

"It gives us the greatest control (over water supply)," said Ken Weinberg, the Water Authority's director of water resources. "And we're less susceptible to rate increases by not being so over-invested" in other agencies that provide water to the region.

At the workshop, Water Authority board members heard a presentation from the agency's staff outlining the comparitive costs to ratepayers for several plans, including a plan that would rely more heavily on importing water from the Metropolitan Water District, which brings water from Northern California and the Colorado River.

Staffers also compared different prices for plans with and without increasing storage and water treatment projects. All plans included rate increases that, by 2016, would range from $2.24 to $3.75 a month to average ratepayers.

An average ratepayer was defined as a family of four consuming one-half of an acre-foot of water -- 162,950 gallon -- a year, or roughly 13,580 gallons a month.

Officials stressed that the $3.75 a month estimate was just the increase of the Water Authority's portion of the water bills, which typically contain charges from three separate agencies: the Metropolitan Water District, which sells water to the Water Authority; the Water Authority; and the local retail water agencies that buy their water from the authority and sell it to local homeowners and businesses.

A review of the master plan, which is expected to be presented to the board members at their May 27 meeting, does not authorize rate increases or construction projects, officials said. Each project must undergo its own environmental review process before construction can begin.

"We're not setting rates," board Chairman Bernie Rhinerson said about the information presented at the workshop. "This is just a planning tool."

Contact staff writer Edward Sifuentes at (760) 740-5426 or esifuentes@nctimes.com.